1. Field of the Invention
This invention relates generally to the dispersing of liquids into fluidized solids. More specifically this invention relates to a method and apparatus for dispersing a hydrocarbon feed into a stream of fluidized catalyst particles.
2. Description of the Prior Art
There are a number of continuous cyclical processes employing fluidized solid techniques in which carbonaceous materials are deposited on the solids in the reaction zone and the solids are conveyed during the course of the cycle to another zone where carbon deposits are at least partially removed by combustion in an oxygen-containing medium. The solids from the latter zone are subsequently withdrawn and reintroduced in whole or in part to the reaction zone.
One of the more important processes of this nature is the fluid catalytic cracking (FCC) process for the conversion of relatively high-boiling hydrocarbons to lighter hydrocarbons boiling in the heating oil or gasoline (or lighter) range. The hydrocarbon feed is contacted in one or more reaction zones with the particulate cracking catalyst maintained in a fluidized state under conditions suitable for the conversion of hydrocarbons.
It has been found that the method of contacting the feedstock with the catalyst can dramatically affect the performance of the reaction zone. Modern FCC units use a pipe reactor in the form of a large, usually vertical, riser in which a gaseous medium upwardly transports the catalyst in a fluidized state. Ideally the feed as it enters the riser is instantaneously dispersed throughout a stream of catalyst that is moving up the riser. A complete and instantaneous dispersal of feed across the entire cross section of the riser is not possible, but good results have been obtained by injecting a highly atomized feed into a pre-accelerated stream of catalyst particles. However, the dispersing of the feed throughout the catalyst particles takes some time, so that there is some non-uniform contact between the feed and catalyst as previously described. Non-uniform contacting of the feed and the catalyst exposes portions of the feed to the catalyst for longer periods of time which can in turn produce overcracking and reduce the quality of reaction products.
It has been a long recognized objective in the FCC process to maximize the dispersal of the hydrocarbon feed into the particulate catalyst suspension. Dividing the feed into small droplets improves dispersion of the feed by increasing the interaction between the liquid and solids. Preferably, the droplet sizes become small enough to permit vaporization of the liquid before it contacts the solids. It is well known that agitation or shearing can atomize a liquid hydrocarbon feed into fine droplets which are then directed at the fluidized solid particles. A variety of methods are known for shearing such liquid streams into fine droplets.
U.S. Pat. No. 3,071,540 discloses a feed injection apparatus for a fluid catalytic cracking unit wherein a high velocity stream of gas, in this case steam, converges around the stream of oil upstream of an orifice through which the mixture of steam and oil is discharged. Initial impact of the steam with the oil stream and subsequent discharge through the orifice atomizes the liquid oil into a dispersion of fine droplets which contact a stream of coaxially flowing catalyst particles.
U.S. Pat. No. 4,434,049 shows a device for injecting a fine dispersion of oil droplets into a fluidized catalyst stream wherein the oil is first discharged through an orifice onto an impact surface located within a mixing tube. The mixing tube delivers a cross flow of steam which simultaneously contacts the liquid. The combined flow of oil and steam exits the conduit through an orifice which atomizes the feed into a dispersion of fine droplets and directs the dispersion into a stream of flowing catalyst particles.
The injection devices of the ""540 and ""049 patents rely on relatively high fluid velocities and pressure drops to achieve atomization of the oil into fine droplets. Providing this higher pressure drop burdens the design and increases the cost of equipment such as pumps and exchangers that are typically used to supply liquid and gas to the feed injection device. The need to replace such equipment may greatly increase the cost of retrofitting an existing liquid-solid contacting installation with such an injection apparatus.
U.S. Pat. No. 4,717,467 shows a method for injecting an FCC feed into an FCC riser from a plurality of discharge points. The discharge points in the ""467 patent do not radially discharge the feed mixture into the riser.
Another useful feature for dispersing feed in FCC units is the use of a lift gas to pre-accelerate the catalyst particles before contact with the feed. Catalyst particles first enter the riser with zero velocity in the ultimate direction of catalyst flow through the riser. Initiating or changing the direction of particle flow creates turbulent conditions at the bottom of the riser. When feed is introduced into the bottom of the riser the turbulence can cause mal-distribution and variations in the contact time between the catalyst and the feed. In order to obtain a more uniform dispersion, the catalyst particles are first contacted with a lift gas to initiate upward movement of the catalyst. The lift gas creates a catalyst pre-acceleration zone that moves the catalyst along the riser before it contacts the feed. After the catalyst is moving up the riser it is contacted with the feed by injecting the feed into a downstream section of the riser. Injecting the feed into a flowing stream of catalyst avoids the turbulence and back mixing of particles and feed that occurs when the feed contacts the catalyst in the bottom of the riser. A good example of the use of lift gas in an FCC riser can be found in U.S. Pat. No. 4,479,870 issued to Hammershaimb and Lomas.
There are additional references which show use of a lift gas in non-catalytic systems. For example, in U.S. Pat. No. 4,427,538 to Bartholic, a gas which may be a light hydrocarbon is mixed with an inert solid at the bottom part of a vertical confined conduit and a heavy petroleum fraction is introduced at a point downstream so as to vary the residence time of the petroleum fraction in the conduit. Similarly, in U.S. Pat. No. 4,427,539 to Busch et al., a C4 minus gas is used to accompany particles of little activity up a riser upstream of charged residual oil so as to aid in dispersing the oil.
Feed atomization, lift-gas and radial injection of feed have been used to more uniformly disperse feed over the cross-section of a riser reaction zone. As feed contacts the hot catalyst, cracking and volumetric expansion of the hydrocarbons causes an increase in the volumetric rate of fluids passing up the riser. A large portion of this volumetric increase occurs immediately downstream of the feed injection point. U.S. Pat. No. 5,562,818 controls the volumetric expansion occurring simultaneously with mixing of catalyst and hydrocarbon feed to avoid the mal-distribution that can adversely affect the quantity and quality of the products obtained from the cracking reaction by eliminating turbulent back mixing as well as quiescent zones in the riser section immediately downstream of the feed injection point.
Further control of feed atomization has been added by U.S. Pat. No. 5,298,155 and U.S. Pat. No. 5,188,805 that show a movable head in nozzle tip to adjust the flow opening and flow characteristics of multiple orifice nozzles. A movable shaft positions the head at variable locations within the orifice throat of the nozzle as the unit operates.
Feed contacting methods have also focused attention on the distribution of catalyst before it contacts the feed. U.S. Pat. No. 5,318,691 provides an extended contacting chamber for adjustment of the catalyst flow regime before a distributor radially injects multiple streams of hydrocarbon feed to form a catalyst and feed mixture that passes through a restricted opening and then upwardly in a riser contacting conduit. U.S. Pat. Nos. 5,205,992, 4,960,502, and 4,729,825 disclose various additional methods for distributing fluidized particles at the bottom of a riser contacting conduit before the particles contact the feed fluid.
As shown by the above prior art, most feed injection apparatus are located near the bottom of a riser conduit that injects the feed into the riser to begin upward transport of the particles through the riser or injects the feed into the riser after some other motive fluid has initiated transport of the solid particles up the riser. As the complexity of feed distribution devices increases, shut down of the process unit and the unavoidable back flow of catalyst that occurs can interfere with the operation of the feed nozzles when operations resume. In particular, catalyst particles, especially in the presence of any condensed residual fluids or initially entering fluids, can agglomerate and plug the relatively small openings of the vanes and orifices that find common use in today""s fluidized particulate processes. Special shutdown procedures can be followed to reduce or eliminate back flow of particles or the presence of particles in the feed distribution devices. However, such procedures may not be possible when an emergency dictates the time frame of a shutdown. Agglomeration that occurs will at least protract start-up time and, in more inconvenient situations, may require time consuming and expensive disassembly of equipment before operations may resume.
It is an object of this invention to provide a method and apparatus that uniformly injects a well dispersed feed into a well dispersed stream of particles and prevents back flow of particles into restricted feed distribution passages.
It is a further object of this invention to provide a method and apparatus that simplifies the adjustment of feed distribution for changes in fluid feed flow rate during the operation of the fluid solid contacting.
These objects are achieved by the use of a fluid feed distributor that uses a flow actuated plug to prevent back flow of fluidizable particles through a fluid feed outlet when fluid feed to the riser conduit ceases. The apparatus is located at or near the bottom of a riser contacting conduit where a back flow of particles can occur. A key function of the apparatus is the at least occluding, and preferably sealing, movement of the flow actuated plug into the fluid feed outlet when fluid flow stops. The apparatus itself will usually have an at least partially vertical orientation to provide the preferred gravity closure of the flow actuated plug, however, suitable resilient devices may be incorporated to provide back flow prevention in horizontal alignments or where additional assistance to gravity closure is desired.
The arrangement can also provide improved feed distribution. Preferably the fluid actuated plug will act in conjunction with a fluid dispersion device. The dispersion device may be located upstream in a feed conduit that houses the fluid outlet at its downstream end. In addition, the fluid actuated plug can also serve to adjust the dispersion of feed fluid during operation of the unit. To enhance feed dispersion the fluid feed outlet will ordinarily provide a restricted opening. A spray nozzle may be incorporated into the restricted opening to improve feed dispersion. A restricted opening in the form of a venturi offers particularly advantageous feed distribution benefits when cooperatively arranged with the plug.
Whether in a venturi shape or not, the nozzle, the opening and the plug may control flow in two areas. In one section the fluid actuated plug may have a similar profile to a shearing surface of the nozzle opening that produces atomization of the feed. The plug can regulate the area at the exit point of the nozzle to maintain high shear action on the feed with regard to its flow rate. The plug may also act to vary a more upstream portion of the nozzle flow area by using a dependent stem as the plug support. The stem will extend through the opening and taper to decreased diameter at its distal end. Upward movement of the plug caused by increased fluid flow, positions increasingly smaller diameter portions of the stem within the opening, thereby increasing the flow area through the opening. The tapered support stem in this manner can again compensate automatically for low flow situations in order to maintain high shear velocities through the outlet.
Accordingly in one embodiment this invention is an apparatus for contacting fluidizable particles with a fluid feed. The apparatus includes an elongated riser conduit having an upper downstream end and a lower upstream end that defines a fluidized catalyst inlet. At least one feed conduit has an outlet surface at least partially defining a restricted fluid outlet and establishing a flow path for a fluid feed into an upstream portion of the riser. In the upstream portion of the riser the fluid feed passes from a feed conduit inlet through a dispersion device and out of the feed conduit through the restricted fluid outlet. A flow actuated plug moves into contact with the outlet surface of the restricted fluid outlet to occlude the restricted fluid outlet and inhibit particle flow in the absence of fluid flow through the feed conduit.
In another embodiment this invention is an apparatus for contacting fluidizable particles with a fluid feed. The apparatus again includes an elongated riser conduit having an upper downstream end and a lower upstream end that defines a fluidized catalyst inlet. At least one feed conduit has a feed inlet at one end, has a feed outlet forming a venturi at an opposite end for injecting the fluid feed into an upstream portion of the riser, and has at least one guide bar extending radially into an intermediate section of the feed conduit. A flow actuated plug is located at least partially above the venturi and has a larger diameter than the opening of the venturi for sealing the venturi from catalyst flow. A stem depends from the bottom of the flow actuated plug and cooperates with the stop bar to limit upward movement of the self actuated plug.
A preferred arrangement for the apparatus of this invention uses a particle distribution zone to provide good distribution of the particles before contacting the feed. Suitable particle distribution arrangements will neutralize the momentum of the particles as they enter the riser by passing them through an annular zone that extends around the outside of the riser conduit. The primary function of the particle distribution zone is to change the direction of the particles for even distribution over a usually annular distribution space that delivers the particles for contact with the feed. In those embodiments where it is used, the particle distribution zone will usually take the form of a particle distribution vessel located at or near the upstream end of the riser conduit and be in communication with a fluidized particle inlet. The particle distribution vessel will define a particle distribution chamber that communicates with a source of particles, will contain at least one inlet for receiving a fluidizing gas that fluidizes particles for distribution around the chamber, and will define an outlet in communication with the fluidized particle inlet.
Additional objects, embodiments and details of this invention can be obtained from the following detailed description.